Video encoding method and video decoding method

ABSTRACT

Provided is a video encoding/decoding technique for improving the compression efficiency by reducing the motion vector code amount. In a video decoding process, the prediction vector calculation method is switched from one to another in accordance with a difference between predetermined motion vectors among a plurality of motion vectors of a peripheral block of a block to be decoded and already decoded. The calculated prediction vector is added to a difference vector decoded from an encoded stream so as to calculate a motion vector. By using the calculated motion vector, the inter-image prediction process is executed.

This is a continuation application of U.S. application Ser. No.13/058,560, filed Feb. 11, 2011, which is a national stage ofPCT/JP2009/002460, filed on Jun. 2, 2009, which claims priority to JP2008-249515, filed Sep. 29, 2008. The entire disclosures of all of theseapplications are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a technique for encoding a video(dynamic image) and a technique for decoding a video (dynamic image).

BACKGROUND ART

As a method for digitalizing, recording and transmitting a large amountof video information, encoding formats such as Moving Picture ExpertsGroup (MPEG) have been established. For example, MPEG-1 format, MPEG-2format, MPEG-4 format H.264/Advanced Video Coding (AVC) format and thelike have been established as international standard encoding formats.These formats are used for digital satellite broadcasting, digitalversatile discs (DVD), mobile phones, digital cameras and the like asencoding formats. The range of use of the formats has been expanded, andthe formats are more commonly used.

According to the formats, an image to be encoded is predicted on a blockbasis using information on an encoded image, and the difference(prediction difference) between an original image and the predictedimage is encoded. In the formats, by removing redundancy of video, theamount of coded bits is reduced. Especially, in inter-prediction inwhich an image that is different from an image to be encoded isreferenced, a block that highly correlates with a block to be encoded isdetected from the referenced image. Thus, the prediction is performedwith high accuracy. In this case, however, it is necessary to encode theprediction difference and the result of detecting the block as a motionvector. Thus, an overhead may affect the amount of coded bits.

In H.264/AVC format, a technique for predicting the motion vector isused in order to reduce the amount of coded bits for the motion vector.That is, in order to encode the motion vector, the motion vector of ablock to be encoded is predicted using an encoded block that is locatednear the block to be encoded. Variable length coding is performed on thedifference (differential motion vector) between the predictive motionvector and the motion vector.

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

However, the accuracy of predicting a motion vector in conventionalH.264/AVC format is not sufficient. A large amount of coded bits for amotion vector are still necessary.

An object of the present invention is to reduce the amount of coded bitsfor a motion vector and improve the efficiency of compression.

Means for Solving the Problem

A video encoding method and a video decoding method according to thepresent invention are provided, for example, as described in claims.

Effect of the Invention

According to the present invention, it is possible to reduce the amountof coded bits for a motion vector and improve the efficiency ofcompression.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a video encoding device according to afirst embodiment of the present invention.

FIG. 2 is a block diagram showing an inter-prediction section includedin the video encoding device according to the first embodiment of thepresent invention.

FIG. 3 is a block diagram showing a video decoding device according tothe first embodiment of the present invention.

FIG. 4 is a block diagram showing an inter-prediction section includedin the video decoding device according to the first embodiment of thepresent invention.

FIG. 5 is a conceptual diagram showing inter-prediction using H.264/AVCformat.

FIG. 6 is a conceptual diagram showing a technique for predicting amotion vector, while the technique is used with H.264/AVC format.

FIG. 7 is a conceptual diagram showing an example of a technique forpredicting a motion vector according to the first embodiment of thepresent invention.

FIG. 8 is a conceptual diagram showing an example of another techniquefor predicting a motion vector according to the first embodiment of thepresent invention.

FIG. 9 is a diagram showing an example of a method for encoding apredictive motion vector according to the first embodiment of thepresent invention.

FIG. 10 is a diagram showing an example of thresholds that are used forthe technique for predicting a motion vector according to the firstembodiment of the present invention.

FIG. 11 is a conceptual diagram showing an example of a code tableswitching technique according to the first embodiment of the presentinvention.

FIG. 12 is a conceptual diagram showing an example of the code tableswitching technique according to the first embodiment of the presentinvention.

FIG. 13 is a diagram showing an example of code tables according to thefirst embodiment of the present invention.

FIG. 14 is a diagram showing an example of the code tables according tothe first embodiment of the present invention.

FIG. 15 is a conceptual diagram showing a technique for predicting amotion vector according to a second embodiment of the present invention.

FIG. 16 is a flowchart of a video encoding method according to the firstembodiment of the present invention.

FIG. 17 is a flowchart of a process of calculating a differential motionvector in the video encoding method according to the first embodiment ofthe present invention.

FIG. 18 is a flowchart of a video decoding method according to the firstembodiment of the present invention.

FIG. 19 is a flowchart of a process of calculating a motion vector inthe video decoding method according to the first embodiment of thepresent invention.

MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention are described with reference to theaccompanying drawings.

FIG. 5 is a conceptual diagram showing an operation of aninter-prediction process according to H.264/AVC format. In H.264/AVCformat, an image to be encoded is encoded on a block basis in order ofraster scanning. In order to perform inter-prediction, an encoded imagethat is included in a video sequence (501) is treated as a referenceimage (502), while the video sequence (501) includes a target image(503) to be encoded. A block (prediction image) (505) highly correlateswith a target block (504) that is included in the target image. Theblock (505) is detected from the reference image. In this case, thedifference between the two blocks is calculated as a predictiondifference. The prediction difference is encoded, while the differencebetween coordinate values of the two blocks is treated as a motionvector (506) and is encoded. In order to perform decoding, in contrast,the aforementioned procedures are performed in the reverse order. Theprediction difference is decoded and then added to the block (predictionimage) (505) included in the reference image, thereby acquiring adecoded image.

In H.264/AVC format, a technique for predicting a motion vector is usedin order to reduce an overhead due to the amount of coded bits for amotion vector as described above. Specifically, in order to encode themotion vector, the motion vector of a block to be encoded is predictedusing an encoded block located near the block to be encoded, and thedifference (differential motion vector) between the predictive motionvector and the motion vector is encoded. In this case, the magnitude ofthe differential motion vector is equal to or nearly equal to 0according to statistics. Thus, the amount of coded bits can be reducedby performing variable length coding on the differential motion vector.FIG. 6 is a conceptual diagram showing a method for calculating thepredictive motion vector. A block A (602) that has been encoded islocated on the left side of a target block (601) to be encoded. A blockB (603) that has been encoded is located on the upper side of the block(601) to be encoded. A block C (604) that has been encoded is located onthe upper right side of the block (601) to be encoded. Motion vectors ofthe blocks A, B and C are indicated by MVA, MVB and MVC, respectively.

In this case, in H.264/AVC format, a predictive motion vector iscalculated as a median of the motion vectors MVA, MVB and MVC.Specifically, the predictive motion vector PMV is calculated using afunction Median (605) that returns the median of components of vectorsspecified as arguments. In addition, a differential motion vector DMV iscalculated as the difference (606) between the motion vector MV of theblock to be encoded and the predictive motion vector PMV. Then, variablelength coding is performed on the differential motion vector DMV.Decoding is performed in the reverse order thereof. Specifically, thedifferential motion vector DMV is decoded and added to the predictivemotion vector PMV calculated in the aforementioned manner, therebydecoding the motion vector MV.

As described above, in H.264/AVC format, the technique for predicting amotion vector is used, so that the amount of coded bits necessary forthe motion vector is significantly reduced. However, when it isdifficult to accurately predict a motion vector (i.e., when multiplemovable objects are located close to each other or when a boundarybetween movable objects is present near a target region), the accuracyof predicting a motion vector according to H.264/AVC format is notsufficient, and a large amount of coded bits for the motion vector arestill necessary. The reason can be considered as follows. That is, whena motion is complicated like the aforementioned circumstances,correlations between motion vectors of blocks located near the targetregion are significantly reduced, and the difference between vectorsthat are candidates for predictive motion vectors is large. Thus, if apredictive motion vector were erroneously selected, the differentialmotion vector would be large compared with the case where an appropriatepredictive motion vector is selected. As a result, the amount of codedbits is significantly increased.

In an embodiment of the present invention, a method for determining apredictive motion vector is switched among methods for determining thepredictive motion vector on the basis of a distribution of the values ofvectors that are candidates for the predictive motion vector. When therange of the distribution of the candidate vectors is small, it isdetermined that a risk existing when a predictive motion vector iserroneously selected is small, and a conventional prediction method isperformed. In contrast, when the range of the distribution of thecandidate vectors is large, a bit (hereinafter referred to as added bit)that represents a candidate vector to be used as the predictive motionvector is added, and the candidate vector is specified so that thedifferential motion vector is minimized.

In this case, when the type of the vector that is a candidate for thepredictive motion vector is dynamically changed on the basis of thedistribution of the candidate vectors, it is possible to suppress anincrease in the amount of coded bits due to the added bit or bits.Therefore, while it is possible to suppress an increase in the amount ofcoded bits, it is possible to improve the accuracy of predicting amotion vector.

In general, when a motion is complicated, the accuracy of predicting amotion vector is reduced. Even when the optimal predictive motion vectoris selected, the differential motion vector is not small. Thus, in orderto reduce the amount of coded bits, it is effective to change, on thebasis of whether or not a motion is complicated, a method for encodingthe differential motion vector.

For example, in Reference Document 1, it is determined whether or not amotion is complicated on the basis of the magnitude of dispersion ofmotion vectors of blocks located near a target block, and a variablelength code table is switched, on the basis of the determination result,among variable length code tables that are used in order to encode adifferential motion vector. In this method, it can be roughly determinedwhether or not a motion is complicated. However, a code table cannot beexactly switched among the tables so that the switching reflectscharacteristics of an image. In addition, the switching of the codetable in this method is performed on the basis of the motion vectors ofthe blocks located near the target block. Thus, when a motion in atarget region is different from a motion in a region located near thetarget region, a code table cannot be appropriately selected.

According to the embodiment of the present invention, in a method forselecting the optimal vector using the aforementioned added bit or bits,a characteristic of a motion in a target region can be predicted indetail by specifying a selected candidate vector, and a code table isswitched among code tables on the basis of the predicted information.Thus, it is possible to more accurately switch a code table among thecode tables. As a result, it is possible to further reduce the amount ofcoded bits.

-   [Reference Document 1] JP-A-2006-271001

A process of encoding a motion vector according to the present inventionand a process of decoding a motion vector according to the presentinvention are described below. A process of calculating a predictivemotion vector PMV in the encoding process is performed in the samemanner as a process of calculating a predictive motion vector PMV in thedecoding process. In the encoding process, the difference (differentialmotion vector) DMV between a motion vector MV and a predictive motionvector PMV is calculated and encoded. In the decoding process, incontrast, the differential motion vector DMV is decoded, the predictivemotion vector PMV is added to the decoded differential motion vectorDMV, and the motion vector MV is decoded.

First Embodiment

FIG. 7 is a conceptual diagram showing an example of the predictivemotion vector calculating method according to the present embodiment. InFIG. 7, vectors that are candidates for a predictive motion vector arevectors (of three types) of blocks A, B and C. The block A is located onthe left side of a target block. The block B is located on the upperside of the target block. The block C is located on the upper right sideof the target block. In this case, a motion vector of the block A isindicated by MVA; a motion vector of the block B is indicated by MVB;and a motion vector of the block C is indicated by MVC.

First, x and y components of the motion vectors MVA, MVB and MVC arealigned. A distribution of the motion vectors is examined using athreshold Thre1 on the basis of four types of cases CASE1 to CASE4. InFIG. 7, the directions of arrows are directions in which the components(of the motion vectors) that have large values extend. Among the x and ycomponents of the motion vectors MVA, MVB and MVC indicated by symbolsx, a component that is closest to a location pointed by each of thearrows has the maximum value, while a component that is farthest fromthe location pointed by each of the arrows has the minimum value. Acomponent located between both the components has an intermediate value.

When an interval between each pair of all the values is smaller than thethreshold Thre1 (CASE1) and any value is selected from among the values,the magnitude of the differential motion vector does not significantlyvary. Thus, in the same manner as H.264/AVC format, a median (a) amongthe candidate values is selected as the predictive motion vector PMV. Inthis case, an added bit is not generated. In this case, it is notnecessary to select the median. For example, the average value, themaximum value, the minimum value or the like may be selected as thepredictive motion vector using any calculation method. A motion vectorof a block other than the blocks A, B and C may be used to determine thepredictive motion vector. For example, a motion vector of a blocklocated on the upper right side of the target block may be used todetermine the predictive motion vector. In addition, the followingmotion vector may be used to determine the predictive motion vector: amotion vector of a block that is located at the same position as thetarget block and is included in a frame that chronologically precedes aframe including the target block.

In contrast, in the case CASE2 in which the difference between themaximum value among the candidate values and the median is equal to orlarger than the threshold Thre1 and the difference between the minimumvalue and the median is smaller than the threshold Thre1, when it isoptimal to select the minimum value as the predictive motion vector andthe median is selected as the predictive motion vector, for example, themagnitude of the differential motion vector does not significantly vary.In the case CASE2, however, if the maximum value is selected when it isnecessary to select the median, the magnitude of the differential motionvector is significantly increased. Thus, in the case CASE2, options forthe predictive motion vector PMV are the maximum value (b) and themedian (c), and the maximum value (b) or the median (c) is selected asthe predictive motion vector PMV so that the selected value results in asmaller differential motion vector. Whether the maximum value or themedian is selected is represented by information of one bit. In thedecoding process, the predictive motion vector is specified on the basisof the one-bit information and the specified predictive motion vector isadded to the differential motion vector, thereby decoding the motionvector.

Similarly, when the difference between the minimum value and a median isequal to or larger than the threshold Thre1, and the difference betweenthe maximum value and the median is smaller than the threshold Thre1(CASES), the median (d) or the minimum value (e) is selected as thepredictive motion vector PMV so that the selected value results in asmaller differential motion vector. Then, one-bit information is added.

When all intervals between the values are equal to or larger than thethreshold Thre1 (CASE4), a value is selected as the predictive motionvector from among three candidate values that are the maximum value (f),a median (g), and the minimum value (h) so that the selected valueresults in the smallest differential motion vector, and information ofone or two bits is added.

The methods for setting the options for the predictive motion vector arenot limited. In the case CASE4, for example, since the number of theoptions is three, the two added bits are necessary in some cases. Theoptions may be limited to the two types of the motion vectors MVA andMVB, for example. In this case, it is always possible to suppress theadded bits to one bit.

In the aforementioned method, the predictive motion vector can berepresented by means of the added bit of the minimum data amount onlywhen it is highly likely that the accuracy of prediction is reduced.Thus, the accuracy of predicting a motion vector can be improved whileit is possible to suppress an increase in the amount of coded bits.

When the aforementioned method is performed together with a methoddescribed below with reference to FIG. 8, the accuracy of the predictioncan be further increased. A threshold Thre2 that is set as a valuelarger than the threshold Thre1 is used. A distribution of the motionvectors is examined using the threshold Thre2 on the basis of threetypes of cases CASE5 to CASE7 as well as the cases CASE1 to CASE4.

Specifically, when the difference between the values (b) and (c) in thecase CASE2 is equal to or larger than the threshold Thre2 (CASES), anintermediate value (i) is added between the values (b) an (c) as anoption for the predictive motion vector. A value is selected as thepredictive motion value from among the values (b), (c) and (i) so thatthe selected value results in the smallest differential motion vector.Information of one or two bits is added.

In addition, when the difference between the values (d) and (e) in thecase CASE3 is equal to or larger than the threshold Thre2 (CASE6), anintermediate value (j) is added between the values (d) and (e) as anoption for the predictive motion vector. A value is selected as thepredictive motion vector from among the values (d), (e) and (j) so thatthe selected value results in the smallest differential motion vector.Information of one or two bits is added.

In addition, when the difference between the values (f) and (g) in thecase CASE4 and the difference between the values (g) and (h) are equalto or larger than the threshold Thre2 (CASE7), an intermediate value (k)is added between the values (f) and (g) as an option for the predictivemotion vector and an intermediate value (l) is added between the values(g) and (h) as an option for the predictive motion vector. A value isselected as the predictive motion vector from among the values (f), (g),(h), (k) and (l) so that the selected value results in the smallestdifferential motion vector. Information of one bit, two bits or threebits is added.

As described above, when an interval between candidate values is large,it is highly likely that the magnitude of a differential motion vectoris increased. Thus, when an intermediate value is added between thecandidate values as a new option, it is highly likely that theprediction is accurately performed. Accordingly, the difference betweenthe predictive motion vector and an actual vector is small, and it ispossible to reduce the amount of coded bits.

In the aforementioned example, the intermediate value is added betweenthe two types of the candidate values as a new option. However, anycalculation method may be performed using candidate values. For example,a weighted mean calculation may be performed using multiple candidatevalues. In addition, a method for adding an option for the predictivemotion vector is not limited. Moreover, in the aforementioned example,the method described with reference to FIG. 7 and the method describedwith reference to FIG. 8 are combined and used. However, each methoddescribed with reference to FIGS. 7 and 8 may be used independently.

FIG. 9 shows a method for encoding the predictive motion vector. FIG. 9shows variable length code tables that are used to encode the values inthe cases CASE2, CASE4 and CASE5. The cases CASE2, CASE4 and CASE5 arerepresentative examples in which the numbers of options are 2, 3 and 5.However, the variable length code tables are examples. A method forgenerating code tables is not limited.

In addition, a method for setting the thresholds Thre1 and Thre2 is notlimited. The thresholds Thre1 and Thre2 may be fixed values. Forexample, when the thresholds are dynamically determined on the basis ofa quantization parameter or the like as shown in FIG. 10, the thresholdsare effective. In this example, the thresholds are set to larger valuesas the quantization parameter is larger. The reason is as follows. Whenthe quantization parameter is large, the bit rate is small and an effectof the added bit or bits is large. Thus, when the thresholds are set tolarge values, it is unlikely that the added bit is generated. Therefore,the thresholds are effective.

In the embodiment of the present invention, the method for encoding adifferential motion vector is switched among methods for encoding thedifferential motion vector on the basis of information selected fromamong the candidate vectors, and then the amount of coded bits isfurther reduced. FIG. 11 shows a method for estimating a characteristicof an image from information selected from among the candidate vectors.For example, when any of the components a, c and d of the candidatevectors is selected for a target block in order to encode and decode apredictive motion vector, it is apparent that a motion vector in atarget region is similar to a vector of neighboring blocks, and it isestimated that the target region is present in a large object. When anyof the components b and e of the candidate vectors is selected, twotypes of motions are present near a target region and it is estimatedthat the target region is located at a boundary portion of the largeobject. In contrast, when any of the components f, g, h, i, j, k and lof the candidate vectors is selected, correlations among motion vectorsin regions located near a target region are low, and it is estimatedthat the target region is present in a non-correlation region that is,for example, a region in which many small objects are present.

FIG. 12 shows the method for switching a variable length code table fora differential motion vector on the basis of estimated information on acharacteristic of an image as described above (information selected fromamong the candidate vectors). In general, when a motion is complicated,the accuracy of predicting a motion vector is reduced. In theaforementioned example, the prediction accuracy is reduced in order of“the region in the object”, “the boundary region of the object” and“non-correlation region”, and the magnitude of the differential motionvector is increased in the above order (1201). In the present invention,multiple variable length code tables (a table A (1202), a table B (1203)and a table C (1204)) are prepared, and a variable length code table isswitched among the multiple variable length code tables on the basis ofthe characteristic of the table (1205). For example, as the table A, thefollowing table is used: a table that results in the fact that when thevalue of the differential motion vector is small, a code length issmall, and when the value of the differential motion vector isincreased, the code length is abruptly increased. As the table C, incontrast, the following table is used: a table that results in the factthat when the value of the differential motion vector is small, the codelength is large, and even when the value of the differential motionvector is increased, the code length is gently increased. Also, as thetable B, a table that has an intermediate property of the tables A and Cis used.

In this case, when the target region is present in the object (or whenany of the components a, c and d of the candidate vectors is selected inorder to encode and decode the predictive motion vector), thedifferential motion vector is encoded using the table A that iseffective when the value of the differential motion vector is small. Incontrast, when the target region is present in the non-correlationregion (or when any of the components f, g, h, i, j, k and l of thecandidate vectors is selected in order to encode and decode thepredictive motion vector), the differential motion vector is encodedusing the table C that is effective when the value of the differentialmotion vector is large. When the target region is located at a boundaryportion of the object (or when any of the components b and e of thecandidate vectors is selected in order to encode and decode thepredictive motion vector), the differential motion vector is encodedusing the table B that has the intermediate property of the tables A andC. In the aforementioned method, it is possible to accurately switch acode table on the basis of a characteristic of a target image andsignificantly reduce the amount of coded bits necessary for thedifferential motion vector.

Although any variable length code table may be used, it is effective touse a table A (1301), a table B (1302) and a table C (1303), which areshown in FIG. 13.

In this manner, the tables A, B and C may be defined as fixed tables inadvance. For example, as shown in FIG. 14, it may be effective thatmultiple tables (tables 1 to 5) may be prepared (1402), and a table isdynamically selected from among the multiple tables on the basis of aparameter. In this example, combinations of table numbers that areassigned to the tables A, B and C are defined as table sets (a, b and c)(1401). A table set to be used is switched among the table sets on thebasis of a cumulative value (PrevAddBits) of the added bit (or bits) fora frame that has been encoded and decoded immediately before a targetimage (1403). When a motion in a target frame is active, and codelengths of the tables A, B and C are largely biased, the effect ofreducing the amount of coded bits is increased. Thus, tables areswitched among the table sets on the basis of the parameter(PrevAddBits) that reflects the magnitude of a motion in the immediatelyprevious frame. In this example, thresholds (Thre3 and Thre4) that areconstant numbers are set in order to determine the switching. The methodfor determining the switching is not limited. In the aforementionedexample, PrevAddBits is used as the parameter for the switching. Aparameter such as the average value of motion vectors, a distributionvalue of the motion vectors, or prediction error statistics may be usedas long as the parameter reflects the amount of a motion in a frame.

FIG. 1 shows an example of a video encoding device according to thepresent embodiment. The video encoding device includes: an input imagememory (102) that holds an original image (101); a block dividingsection (103) that divides the input image into small regions; anintra-prediction section (105) that performs intra-prediction on a blockbasis; an inter-prediction section (106) that performs inter-predictionon a block basis on the basis of the amount of a motion detected by amotion searching section (104); a mode selecting section (107) thatdetermines prediction encoding means (prediction method and block size)that are suitable for a characteristic of an image; a subtractingsection (108) that generates a prediction difference; a frequencytransforming section (109) and a quantization section (110) that performencoding on the prediction difference; a variable length coding section(111) that performs encoding on the basis of the possibility ofgeneration of a code; an inverse quantization section (112) and aninverse frequency transforming section (113) that decode the encodedprediction difference; an adding section (114) that generates a decodedimage using the decoded prediction difference; and a reference imagememory (115) that holds the decoded image and uses the decoded image forsubsequent prediction.

The input image memory (102) holds, as an image to be encoded, a singleimage included in the original image (101). The image to be encoded isdivided into small blocks by the block dividing section (103). The blockdividing section (103) transmits the blocks to the motion searchingsection (104), the intra-prediction section (105) and theinter-prediction section (106). The motion searching section (104)calculates the amount of a motion of each of the blocks using thedecoded image stored in the reference image memory (115). The motionsearching section (104) transmits motion vectors to the inter-predictionsection (106). While the blocks are classified into some block sizes,the intra-prediction section (105) performs intra-prediction on theblocks on a block size basis and the inter-prediction section (106)performs inter-prediction on the blocks on a block size basis. The modeselecting section (107) selects the optimal prediction method from amongthe intra-prediction and the inter-prediction. Subsequently, thesubtracting section (108) generates a prediction difference according tothe optimal prediction encoding means and transmits the generatedprediction difference to the frequency transforming section (109). Thefrequency transforming section (109) performs frequency transform suchas discrete cosine transformation (DCT) on the transmitted predictiondifference on a specified block size basis. Then, the quantizationsection (110) performs a quantization process on the transmittedprediction difference on the specified block size basis and transmitsthe prediction difference to the variable length coding section (111)and the inverse quantization section (112). The variable length codingsection (111) performs variable length coding on prediction differenceinformation represented by a frequency transform coefficient and oninformation necessary for decoding on the basis of the possibility ofgeneration of a code, and generates a coded stream. In this case, theinformation that is necessary for decoding is, for example, a predicteddirection used to perform the intra-prediction, a motion vector used toperform the inter-prediction, and the like. In the variable lengthcoding process that is performed by the variable length coding section(111), a switching process is performed to select a variable length codetable from among the variable length code tables shown in FIGS. 9, 11,12, 13 and 14, for example. The inverse quantization section (112)performs inverse quantization on the quantized frequency transformcoefficient. Then, the inverse frequency transforming section (113)performs inverse frequency transform such as inverse DCT (IDCT) on thefrequency transform coefficient and acquires the prediction difference.Then, the inverse frequency transforming section (113) transmits theprediction difference to the adding section (114). Subsequently, theadding section (114) generates a decoded image. The decoded image isstored in the reference image memory (115).

FIG. 2 shows an example of details of the inter-prediction section(106). The inter-prediction section includes: a motion vector storagememory (201) that stores motion vectors in encoded regions; a predictivemotion vector calculating section (202) that calculates a predictivemotion vector using the motion vectors in the encoded regions; asubtractor (203) that calculates a differential motion vector bycalculating the difference between a motion vector and the predictivemotion vector; a prediction image generating section (204) thatgenerates a prediction image; and a code table switching section (205)that selects an optimal variable length code table on the basis ofinformation selected from among predictive motion vectors.

The predictive motion vector calculating section (202) calculates apredictive motion vector of a target block on the basis of the motionvectors (in the encoded regions) stored in the motion vector storagememory (201). The process of calculating the predictive motion vector isdescribed above with reference to FIGS. 7 and 8. The subtractor (203)calculates a differential motion vector (207) by calculating thedifference between the motion vector calculated by the motion searchingsection (104) and the predictive motion vector. The code table switchingsection (205) selects an optimal variable length code table and outputsa code table number (206) of the optimal variable length code table tothe variable length coding section (111). The prediction imagegenerating section (204) generates a prediction image (208) from themotion vector and a reference image. The motion vector is stored in themotion vector storage memory (201).

FIG. 3 shows an example of a video decoding device according to thepresent embodiment. For example, the video decoding device includes: avariable length decoding section (302) that performs variable lengthdecoding on a coded stream (301) generated by the video encoding deviceshown in FIG. 1 so that processes of the variable length decoding areperformed in inverse order of the processes of the variable lengthencoding; an inverse quantization section (303) and an inverse frequencytransforming section (304) that are configured to decode a predictiondifference; an inter-prediction section (305) that performsinter-prediction; an intra-prediction section (306) that performsintra-prediction; an adding section (307) that acquires a decoded image;and a reference image memory (308) that temporarily stores the decodedimage.

The variable length decoding section (302) performs variable lengthdecoding on the coded stream (301) and acquires a frequency transformcoefficient component of a prediction difference and information that isnecessary for a prediction process. The information that is necessaryfor the prediction process includes a block size, a motion vector andthe like.

In the variable length decoding process, the variable length decodingsection (302) acquires decoded motion vectors of peripheral blocks fromthe motion vector storage memory (401) included in the inter-predictionsection (305) described later, and aligns the candidate vectors shown inFIGS. 7 and 8. The variable length decoding section (302) calculatesdifferences between the candidate vectors and determines a distributionof the candidate vectors from among the cases (CASE1 to CASE7). Thevariable length decoding section (302) selects a variable length codetable from among the variable length code tables shown in FIG. 9 on thebasis of the result of determining the distribution (from among thecases CASE1 to CASE7). The variable length decoding section (302) usesthe selected variable length code table shown in FIG. 9 so as todetermine an option indicated by the added bit (or bits) included in thecoded stream. The variable length decoding section (302) uses the optionindicated by the added bit (or bits) to select any of the variablelength code tables shown in FIGS. 12, 13 and 14. In addition, thevariable length decoding section (302) uses the selected variable lengthcode table to perform the variable length decoding process on thedifferential motion vector.

Then, the variable length decoding section (302) transmits the formerinformation on the prediction difference to the inverse quantizationsection (303). The variable length decoding section (302) transmits thelatter information necessary for the prediction process to theinter-prediction section (305) or the intra-prediction section (306).Subsequently, the inverse quantization section (303) performs inversequantization on the information on the prediction difference and theinverse frequency transforming section (304) performs inverse frequencytransform on the information on the prediction difference, therebyperforming decoding. Then, the inter-prediction section (305) or theintra-prediction section (306) references the reference image memory(308) on the basis of the information transmitted by the variable lengthdecoding section (302), and performs the prediction process. The addingsection (307) generates a decoded image and causes the decoded image tobe stored in the reference image memory (308).

FIG. 4 shows an example of details of the inter-prediction section(305). The inter-prediction section includes: a motion vector storagememory (401) that stores motion vectors in decoded regions; a predictivemotion vector calculating section (402) that calculates a predictivemotion vector using the motion vectors in the decoded regions; an adder(403) that calculates a motion vector by calculating the sum of thedifferential motion vector and the predictive motion vector; and aprediction image generating section (404) that generates a predictionimage.

The predictive motion vector calculating section (402) calculates apredictive motion vector of a target block on the basis of the motionvectors (in the decoded regions) stored in the motion vector storagememory (401). The process of calculating the predictive motion vector isdescribed above with reference to FIGS. 7 and 8. The adder (403)calculates the sum of the differential motion vector decoded by thevariable length decoding section and the predictive motion vector tothereby decode the motion vector. The decoded motion vector is stored inthe motion vector storage memory (401), while the prediction imagegenerating section (404) generates a prediction image (405) from themotion vector and the reference image.

FIG. 16 shows steps of a process of encoding a single frame according tothe present embodiment. The following process is performed on all blocks(1601) included in the frame to be encoded. Specifically, prediction isperformed on a corresponding block in all encoding modes (combinationsof prediction methods and block sizes) (1602). The intra-prediction(1604) or the inter-prediction (1605) is performed on the basis of theprediction method (1603), thereby calculating the prediction difference(difference image). In the inter-prediction, the prediction difference(difference image) and a motion vector are encoded. In this case, thedifferential motion vector DMV is calculated (1606) on the basis of thepredictive motion vector PMV calculated using the methods shown in FIGS.7 and 8. Then, a frequency transforming process (1607) is performed onthe prediction difference, and then a quantization process (1608) isperformed on the prediction difference. Then, the variable length codingprocess (1609) is performed using the variable length coding tablesshown in FIGS. 9, 11, 12, 13 and 14 and the process of switching thevariable length coding tables, and then distortion of an image and theamount of coded bits in each of the encoding modes are calculated. Whenthe aforementioned process is completed in each of all the encodingmodes, a mode in which the encoding efficiency is highest is selectedfrom among the encoding modes on the basis of the results of the process(1610). It is possible to select a mode in which the encoding efficiencyis highest from among the encoding modes when RD-optimization scheme isused. In RD-optimization scheme, the optimal encoding mode is selectedon the basis of the relationship between the distortion of the image andthe amount of the coded bits. For details of RD-optimization scheme,refer to Reference Document 2.

[Reference Document 2] G. Sullivan and T. Wiegand: “Rate-DistortionOptimization for Video Compression”, IEEE Signal Processing Magazine,vol. 15, no. 6, pp. 74-90, 1998.

Subsequently, an inverse quantization process (1611) and an inversefrequency transforming process (1612) are performed on the quantizedfrequency transform coefficient in the selected encoding mode, and thenthe prediction difference is decoded and a decoded image is generated.The decoded image is stored in the reference image memory (1613). Theaforementioned process is performed on all the blocks. Then, theencoding of the single image frame is completed (1614).

FIG. 17 shows steps of a process (1606) (shown in FIG. 16) ofcalculating the differential motion vector DMV in detail. First, vectors(candidate vectors) of blocks located near a target block are aligned(1701). In the present embodiment, the “alignment” means that themaximum value, a median and the minimum value are calculated from valuesof components of the multiple candidate vectors in a predetermineddirection, and a vector with the maximum value, a vector with the medianand a vector with the minimum value are determined. Alignment describedbelow applies the same. It is determined whether or not intervalsbetween the values of the components of the multiple candidate vectorsin the predetermined direction are equal to or larger than the thresholdThre1 (1702). When the intervals between the values of the components ofthe multiple candidate vectors in the predetermined direction aresmaller than the threshold Thre1, a predictive motion vector PMV iscalculated using the candidate vector that has the median (1703) in asimilar manner to the conventional prediction method. In contrast, whenany of the intervals between the values of the components of themultiple candidate vectors in the predetermined direction is equal to orlarger than the threshold Thre1, it is determined whether or not theintervals between the values of the components of the multiple candidatevectors in the predetermined direction are equal to or larger than thethreshold Thre2 (1704). When the intervals between the values of thecomponents of the multiple candidate vectors in the predetermineddirection are smaller than the threshold Thre2, a candidate vector isselected as the predictive motion vector PMV from among the candidatevalues that are options so that the differential motion vector issmallest (1705). The selected information is added as the added bit orbits (1706). In contrast, when any of the intervals between the valuesof the components of the multiple candidate vectors in the predetermineddirection is equal to or larger than the threshold Thre2, anintermediate value among the candidate values is calculated, and anotheroption for the prediction value is generated (1707). Then, a candidatevector is selected as the predictive motion vector PMV from among thecandidate values that are options so that the differential motion vectoris smallest (1705). The selected information is added as the added bitor bits (1706). The predictive motion vector PMV is calculated by theaforementioned process. After that, the difference between the motionvector MV and the predictive motion vector PMV is calculated as thedifferential motion vector DMV (1710). A code table for the differentialmotion vector DMV is selected on the basis of the selected predictivemotion vector PMV (1710). When the aforementioned process is completed,the process of calculating the differential motion vector DMV iscompleted (1711). In the aforementioned process, the process ofcalculating the predictive motion vector corresponds to the process(shown in FIGS. 7 and 8) of calculating the predictive motion vector.

FIG. 18 shows steps of a process of decoding a single frame according tothe present embodiment. The following process is performed on each ofall the blocks of the single frame (1801). Specifically, the variablelength decoding process is performed on an input stream, and then thefrequency transform coefficient component of the prediction differenceand the differential motion vector are decoded (1802).

In the variable length decoding process, motion vectors of decodedblocks located near the target block are acquired and the candidatevectors shown in FIGS. 7 and 8 are aligned. Then, differences betweenthe candidate vectors are calculated. A distribution of the candidatevectors is determined from among the cases CASE1 to CASE7. A variablelength code table is selected from among the variable length code tablesshown in FIG. 9 on the basis of the result of determining thedistribution (any of the cases CASE1 to CASE7). An option indicated bythe added bit (or bits) included in the coded stream is identified usingthe selected code table shown in FIG. 9. A variable length code table isselected from among the variable length code tables shown in FIGS. 12,13 and 14 using the option indicated by the added bit or bits. Inaddition, the variable length decoding process is performed on thedifferential motion vector using the selected variable length codetable.

Next, the inverse quantization process (1803) and the inverse frequencytransforming process (1804) are performed on the frequency transformcoefficient component of the prediction difference acquired in thevariable length decoding process, and then the prediction difference(differential image) is decoded. Subsequently, the intra-prediction(1806) and the inter-prediction (1808) are performed on the basis of theprediction method (1805). Before the inter-prediction is performed, themotion vector MV is decoded. The differential motion vector DMV isdecoded in the variable length decoding process (1802). The differentialmotion vector DMV and the predictive motion vector PMV calculated by themethods shown in FIGS. 7 and 8 are summed, and the motion vector MV iscalculated (1807). The inter-prediction process (1808) is performedusing the calculated motion vector MV. The aforementioned process isperformed on each of all the blocks of the frame. Then, the decoding ofthe single image frame is completed (1809).

FIG. 19 shows steps of the process (1807) (shown in FIG. 18) ofcalculating the motion vector MV in detail. First, vectors (candidatevectors) of blocks located near the target block are aligned (1901). Itis determined whether or not intervals between the values of thecomponents of the candidate vectors in the predetermined direction areequal to or larger than the threshold Thre1 (1902). When the intervalsbetween the values of the components of the candidate vectors in thepredetermined direction are smaller than the threshold Thre1, apredictive motion vector PMV is calculated using the candidate vectorthat has the median (1903) in a similar manner to the conventionalprediction method. In contrast, when any of the intervals between thevalues of the components of the candidate vectors in the predetermineddirection is equal to or larger than the threshold Thre1, it isdetermined whether or not the intervals between the values of thecomponents of the candidate vectors in the predetermined direction areequal to or larger than the threshold Thre2 (1904). When the intervalsbetween the values of the components of the candidate vectors in thepredetermined direction are smaller than the threshold Thre2, the addedbit (or bits) is read and a value selected as the predictive motionvector PMV is specified so that the predictive motion vector PMV isdecoded (1905). When any of the intervals between the values of thecomponents of the multiple candidate vectors in the predetermineddirection is equal to or larger than the threshold Thre2, anintermediate value among the candidate values is calculated, and anotheroption for the prediction value is generated (1906). Subsequently, theadded bit (or bits) is read and the value selected as the predictivemotion vector PMV is specified, thereby decoding the predictive motionvector PMV (1907). The predictive motion vector PMV is calculated by theaforementioned process. After that, the sum of the predictive motionvector PMV and the differential motion vector DMV is calculated as themotion vector MV (1908). Then, the calculation of the motion vector MVis completed (1909).

In the present embodiment, the predictive motion vector is calculated ona block basis. However, for example, the predictive motion vector may becalculated on an object basis, while objects are separated from abackground of an image. In addition, DCT is used as an example offrequency transform. However, any orthogonal transform (such as discretesine transformation (DST), wavelet transformation (WT), discrete Fouriertransformation (DFT), or Karhunen-Loeve transformation (KLT)) that isused for removal of inter-pixel correlation may be used as frequencytransform. In addition, the predictive difference may be encoded withoutfrequency transform. Furthermore, variable length coding may not beperformed.

In the first embodiment, the three types of peripheral vectors are usedfor the target block as the candidate values for the predictive motionvector. The number of candidate values is not limited. Four or moretypes of peripheral vectors may be used as candidate values.

In the video encoding device, the video encoding method, the videodecoding device and the video decoding method according to the firstembodiment of the present invention described above, it is possible toachieve a video encoding method and a video decoding method in which theamount of coded bits for a motion vector is reduced and the compressionefficiency is improved.

Second Embodiment

A second embodiment of the present invention is described below.

In the first embodiment, the number of vectors used as candidate ofpredictive motion vectors is three. In the second embodiment, as asimpler method, the number of vectors used as candidate of predictivemotion vectors is two.

A video encoding device according to the second embodiment is differentfrom the video encoding device (shown in FIGS. 1 and 2) according to thefirst embodiment only in the method for calculating a predictive motionvector PMV. Thus, other configurations and operations of the videoencoding device according to the second embodiment are described abovein detail, and a description thereof is omitted.

In addition, a video decoding device according to the second embodimentis different from the video decoding device (shown in FIGS. 3 and 4)according to the first embodiment only in the method for calculating apredictive motion vector PMV. Thus, the other configurations andoperations of the video decoding device according to the secondembodiment are described above in detail, and a description thereof isomitted.

In addition, a video encoding method according to the second embodimentis different from the video encoding method (shown in FIG. 16) accordingto the first embodiment only in a method for calculating a differentialmotion vector DMV. Thus, other processes are described above in detailand a description thereof is omitted.

In addition, a video decoding method according to the second embodimentis different from the video decoding method (shown in FIG. 18) accordingto the first embodiment only in a method for calculating a motion vectorMV. Thus, other processes are described above in detail and adescription thereof is omitted.

The method for calculating a predictive motion vector PMV according tothe second embodiment is described below with reference to FIG. 15. FIG.15 is a conceptual diagram showing an example of the method forcalculating a predictive motion vector PMV according to the presentembodiment. In this example, as candidate of predictive motion vectors,two types of the following vectors are used: a motion vector of a blockA located on the left side of a target block; and a motion vector of ablock B located on the upper side of the target block. In this case, themotion vector of the block A is indicated by MVA, while the motionvector of the block B is indicated by MVB. In this example, in order tocalculate a predictive motion vector, a block C (a motion vector MVC)that is located on the upper right side of the target block and anotherblock may be used.

First, x and y components of the motion vectors MVA and MVB arecompared. When the difference between values of the motion vectors MVAand MVB is equal to or lower than the threshold Thre1, and any of thevalues is selected, the differential motion vector does notsignificantly vary. Thus, in a similar manner to H.264/AVC format, themedian of the motion vectors MVA and MVB is selected as the predictivemotion vector PMV (1501). In this case, an added bit is not generated.In this case, the median may not be used, and the average value, themaximum value, the minimum value or the like may be used for thecalculation. In addition, in this case, a motion vector (such as themotion vector of the block located on the upper right side of the targetblock, a motion vector of a block that is located at the same positionas the target block and included in a frame that chronologicallyprecedes a frame that includes the target block) of a block other thanthe block A and B may be used.

When the difference between the values of the motion vectors MVA and MVBis a value between the threshold Thre1 and the threshold Thre2, the twomotion vectors MVA and MVB are options for the predictive motion vector,and any of the motion vectors MVA and MVB is selected as the predictivemotion vector PMV so that the differential motion vector is smaller.One-bit information is added. When the difference between the values ofthe motion vectors MVA and MVB is equal to or larger than the thresholdThre2, the three motion vectors MVA, MVB and (MVA+MVB)/2 are options forthe predictive motion vector, and any of the three motion vectors isselected as the predictive motion vector PMV so that the differentialmotion vector is smallest. Information of one or two bits is added.

In the video encoding device and the video encoding method according tothe second embodiment, the differential motion vector DMV is calculatedby calculating the difference between the motion vector MV calculated bythe inter-prediction and the predictive motion vector PMV calculated asdescribed above, and the video encoding process is performed.

In the video decoding device and the video decoding method according tothe second embodiment, the motion vector MV is calculated by adding thedifferential motion vector DMV decoded from the coded stream to thecalculated predictive motion vector PMV, the inter-prediction process isperformed on the motion vector MV, and the video decoding process isperformed.

In the present embodiment, the predictive motion vector is calculated ona block basis. The predictive motion vector may be calculated on anobject basis, while objects are separated from a background of an image.In addition, DCT is used as an example of frequency transform. However,any orthogonal transform (such as discrete sine transformation (DST),wavelet transformation (WT), discrete Fourier transformation (DFT), orKarhunen-Loeve transformation (KLT)) that is used for removal ofinter-pixel correlation may be used as frequency transform. In addition,the prediction difference may be encoded without frequency transform.Furthermore, variable length coding may not be performed.

In the video encoding device, the video encoding method, the videodecoding device and the video decoding method according to the secondembodiment of the present invention, it is possible to simplify theprocesses and reduce the throughput in addition to the effects of thefirst embodiment.

DESCRIPTION OF REFERENCE NUMERALS

-   102 . . . Input image memory, 103 . . . Block dividing section, 104    . . . Motion searching section, 105 . . . Intra-prediction section,    106 . . . Inter-prediction section, 107 . . . Mode selecting    section, 108 . . . Subtracting section, 109 . . . Frequency    transforming section, 110 . . . Quantization section, 111 . . .    Variable length coding section, 112 . . . Inverse quantization    section, 113 . . . Inverse frequency transforming section, 114 . . .    Adding section, 115 . . . Reference image memory, 302 . . . Variable    length decoding section, 303 . . . Inverse quantization section, 304    . . . Inverse frequency transforming section, 305 . . .    Inter-prediction section, 306 . . . Intra-prediction section, 307 .    . . Adding section, 308 . . . Reference image memory

The invention claimed is:
 1. A video decoding method comprising thesteps of: calculating a motion vector based on information decoded froma coded stream; and performing an inter-prediction process using themotion vector calculated in the motion vector calculating step, whereinthe motion vector calculating step includes: a first process to select aplurality of candidate blocks including decoded blocks located near ablock to be decoded, as options to be used for calculating a motionvector of the block to be decoded, and exclude one of the candidateblocks from the options when a difference between motion vectors of thecandidate blocks located near the block to be decoded is smaller than athreshold value; a second process to read out selection informationindicating which one of the options is to be selected among the optionsfrom the coded stream; and a third process to calculate the motionvector for the inter-prediction process based on the selectioninformation.